Method for separating metals such as zirconium and hafnium

ABSTRACT

The invention concerns a method for separating a metal ( 1 ) from a metal ( 2 ), preferably zirconium from hafnium, which consists in dissolving said metals in an aqueous solution wherein said metals are in a state preventing them from passing through a nanofiltration membrane; treating the aqueous medium with a ligand, for example EDTA, which is complexed with metal ( 1 ) and/or metal ( 2 ), then in passing the resulting treated medium on a filtering membrane allowing through the ligand-metal complexes, but retaining the metals not complexed with the ligand.

[0001] The present invention relates to a process which makes possiblethe separation of certain metals, in particular the separation ofzirconium and hafnium.

[0002] Zirconium has a very low neutron capture cross section and, forthis reason, is used in nuclear reactors. The ore zircon, however,always contains hafnium, in a proportion of approximately 1 to 3% byweight. In contrast to zirconium, hafnium strongly absorbs neutrons. Theuse of zirconium in the nuclear field therefore requires the preliminaryremoval of hafnium, a content of less than 100 ppm often beingrecommended.

[0003] Zirconium has other applications in which its purification isdesirable. For example, the removal of hafnium is also sought instainless superalloys.

[0004] Hafnium and zirconium have very similar properties and theirseparation is thereby rendered extremely difficult. To do this, acarbochlorination of the ore is carried out, which produces thetetrachlorides ZrCl₄ and HfCl₄, and then a separation, either byextractive distillation of the two tetrachlorides or by liquid-liquidextraction after dissolving the chlorides, is carried out. In the lattercase, this results in the formation of ZrO₂ and HfO₂, which thenrequires a further carbochlorination of the zirconium before moving ontothe phase of recovery of the metal Zr. The first process, which is moreefficient, consists of a distillation of molten salts which makes itpossible to retain the tetrachloride form, thus dispensing with thesecond carbochlorination. The temperature is generally comprised between250 and 550° C. See, for example, FR-A-2 250 707 and U.S. Pat. No.4,021,531.

[0005] It is an object of the invention to provide a novel process whichmakes it possible to efficiently separate zirconium from hafnium.

[0006] Another object of the invention is to provide such a processwhich can be applied to the separation of zirconium and hafnium from thestarting ore or from any fraction.

[0007] Another object of the invention is to provide such a processwhich operates at lower temperatures than the prior processes and withreduced operating costs.

[0008] Yet another object of the invention is to provide such a processwhich makes it possible to separate zirconium from hafnium and to exceedthe degrees of purity obtained by the prior processes.

[0009] Yet another object of the invention is to be able to apply thisprocess to the separation of other metals.

[0010] F. Chitry et al. (J. Radioanalytical and Nuclear Chemistry, 1999,240, 2, 931-934) describe the separation of gadolinium and lanthanum bya nanofiltration-complexing in aqueous medium method. This method isbased on the selective complexing of a ligand of the EDTA family, sayDTPA. The DTPA preferentially complexes the gadolinium, which allows theauthors to separate these metals over a nanofiltration membrane, whichretains the gadolinium-DTPA complexes and allows the noncomplexedlanthanum to pass.

[0011] The inventors have tried to transfer the teaching of thesestudies to the separation of zirconium and hafnium. Acidic pH conditionswere adopted under which zirconium and hafnium and their complexes withcompounds of the EDTA type are soluble. A first nanofiltrationexperiment with zirconium and hafnium salts and without use of ligandwas carried out at a pH comprised between 2 and 5. The membrane used hada cutoff threshold of 500 g/mol. The inventors found, with surprise, aretention of zirconium and hafnium of greater than 99.9%, whereaszirconium and hafnium have respective molar masses of 91 g/mol and 179g/mol, much lower than the cutoff threshold of the membrane used (seeexample 1).

[0012] The Applicant Company hypothesizes that zirconium and hafnium,once hydrolyzed in the aqueous phase, have a tendency to form polymersand/or copolymers of the —(O—M(OH)₂)_(n)— type, the molar mass of whichis probably greater than the cutoff threshold of the nanofiltrationmembranes, which excludes them from being used and excludes theapplication of the method disclosed in the above document.

[0013] The Applicant Company has subsequently found, surprisingly, thatit is possible to separate zirconium and hafnium by using acomplexing-filtration method operating according to an entirelydifferent principle.

[0014] The Applicant has also found that this method can be extended tothe separation of metals capable of behaving in aqueous medium in asimilar way to zirconium and to hafnium and, hypothetically, of formingpolymers with relatively high molecular masses. By definition, theprocess applies to metals which are soluble in an aqueous medium butwhich behave in such a medium in such a way that they cannot passthrough a nanofiltration membrane (e.g., a nanofiltration membranehaving a cutoff threshold comprised between 200 and 2 000 g/mol, forexample 1 000 g/mol), which hypothetically will be due to theabovementioned polymeric state.

[0015] A subject matter of the present invention is therefore a processfor separating a metal 1 from a metal 2, starting from a solution ofthese metals in an aqueous medium, e.g. water, in which solution thesemetals are in a state which does not allow them to pass through ananofiltration membrane, in which process the aqueous medium is treatedwith a ligand which complexes with the metal 1 and/or with the metal 2,for example with zirconium and/or with hafnium, and then the medium,thus treated, is passed over a filtration membrane which allowsligand/metal complexes to pass but which retains the metal not complexedwith the ligand. The ligand used has a complexation constant with themetal, e.g. with Zr and Hf, which is sufficiently high to “break” thepolymers and thus to form ligand-metal complexes, the molecular mass ofwhich is lower than that of the polymers and than the cutoff thresholdof the filtration membrane.

[0016] Mention may be made, as metals targeted by the invention, of: Zr,Hf, Al, Ti or Si. It can thus be a matter of the Zr—Ti or Al—Si pairsand more particularly of the Zr—Hf pair.

[0017] In solution, the metals are in the form of salts. These salts canbe of various types, for example chlorates, e.g. MOCl₂, perchlorates ornitrates, e.g. MO(NO₃)₂ (M=metal, e.g. Zr or Hf).

[0018] The aqueous medium in which the metals are in solution and in thepolymeric state is preferably at acidic pH and more particularly athighly acidic pH. The pH depends, of course, on the metals treated.Typically, the top limit is the pH at which the metals begin toprecipitate. The bottom limit may be imposed by the resistance of thefiltration membrane to the acidic conditions and by the behavior of themetals. Generally, the pH will be less than or equal to 4, in particularbetween 1 and 4, preferably between 2 and 4 (in all the ranges indicatedhere, the limits are included). These ranges of values are perfectlyappropriate for the separation of Zr and Hf.

[0019] Filtration membrane and ligand are chosen as a function of oneanother. More specifically, the cutoff threshold of the membrane must besuch that the ligand-metal complex can pass through the membrane.Furthermore, it is obvious that the cutoff threshold of the membranemust be such that the polymer formed by the metals in aqueous mediumcannot pass through the membrane. Routine tests make it possible for aperson skilled in the art to select the best ligand/membrane compromise.

[0020] Use may in particular be made, as filtration membrane, ofnanofiltration membranes having cutoff thresholds of between 200 and 2000 g/mol (they can thus be used for ligand-metal complexes having amolar mass which allows them to pass through the above-defined membrane,for example with a molar mass comprised between approximately 200 and 2000 g/mol) and ultrafiltration membranes having cutoff thresholds ofgreater than 2 000 g/mol.

[0021] By way of example, EDTA forms, with zirconium and hafnium,complexes having a size comprised between 400 and 500 g/mol, and ananofiltration membrane having a cutoff threshold of approximately 1 000g/mol proves to be suitable.

[0022] As regards the ligand, the ideal is to use a ligand specific forone of the two metals to be separated which results in an optimum degreeof separation. In the case of zirconium and hafnium, such a ligandcomplexes with zirconium or with hafnium. Use may also be made of aligand capable of complexing both metals, e.g. hafnium and zirconium,but with different complexation constants, which makes it possible torecover by filtration a first fraction comprising the ligand-metalcomplex having the highest complexation constant. As will be seen later,the amount of ligand selected is then chosen in order to have the bestdegree of separation.

[0023] The ligand has to be soluble in water, as well as the complexeswhich it forms with the metal in solution. The ligand is preferablyorganic.

[0024] It preferably relates to compounds of poly(amino acid) type, inparticular those corresponding to the formula (1):

[0025] in which:

[0026] n=0 to 3,

[0027] m=1 to 4, preferably m=2,

[0028] X and Y are identical or different and represent hydrophilicradicals, in particular of OH or NR¹R² type, with R¹ and R², which areidentical or different, each corresponding to hydrogen or to ahydrophilic monovalent radical preferably selected from aminated and/or(poly)hydroxylated and/or (poly)etherified hydrocarbonaceous residues,these residues preferably being of the cycloalkyl, aralkyl, alkylaryl,cycloalkenyl, aralkenyl, alkenylaryl or aryl type, the number of carbonatoms of which can vary within wide proportions: the molecular mass ofthe compound results therefrom, and therefore the choice of a filtrationmembrane having a cutoff threshold greater than this molecular mass;generally, these residues will have in particular from 2 to 50 carbonatoms, preferably from 4 to 25;

[0029] and cyclic poly(amino acid)s, such as, for example, cyclicpolyaminocarboxylates, such as DOTA:

[0030] Mention may in particular be made, as ligand corresponding tothis definition, of:

[0031] EDTA or ethylenediaminetetraacetic acid

[0032] M=292 g/mol

[0033] diamido-EDTA

[0034] M=290 g/mol

[0035] Diamido-EDTA can be produced from commercial EDTA anhydride (e.g.Aldrich) by reacting NH₃ in aqueous medium. It can also be producedaccording to the method described in Roy P. Houghton and Williams Emyr;JCPRB4; J. Chem. Soc. Perkin Trans. 1, EN, 11, 1982, 2693-2696.

[0036] In the first stage of the process according to the invention, thewater-soluble ligand according to the invention is added to the aqueousmedium to be treated. The amount of ligand added is preferably such thatthe ligand concentration/concentration of metal having the highestcomplexation constant ratio results in the best degree of separation(information accessible by routine tests); the ratio is in particularcomprised between 0.5 and 2, more particularly between 0.8 and 1.7,preferably between 0.9 and 1.6, typical values being, for example, equalto or in the vicinity of 1 or 1.5. Generally, these ligands formcomplexes of 1:1 type.

[0037] It has been seen that the process according to the invention iscarried out on metal salts in solution in an aqueous medium. The processaccording to the invention can thus also comprise, when this isnecessary or desired, an initial stage which makes it possible toprepare such a solution. To do this, use may be made of any known methodfor obtaining such a solution of metal salts. For example, treatmentwith nitric acid in an aqueous medium, e.g. according to U.S. Pat. No.2,285,443 or GB-A-555988.15, can be carried out, to give MO(NO₃)₂ salts.

[0038] In conventional processes for the production of zirconium metal,the zirconium and the hafnium are encountered essentially in the form oftetrachlorides. They are encountered, for example, in this form at theend of the carbochlorination of zircon. The mixture of thetetrachlorides can be directly dissolved in an aqueous medium, e.g.water, at acidic pH, which gives MOCl₂ (M for Zr or Hf) salts. In analternative form, the mixture of tetrachlorides can be treated withnitric acid, as indicated above.

[0039] In other conventional processes for the production of zirconiummetal, the zirconium and the hafnium are encountered in the oxide formMO₂. Here again, use may be made of the treatment with nitric acid. Sucha reaction can be written, for example, as follows:

ZrO₂,xH₂O+HNO₃→Zr(NO₃)_(4,) 5H₂O→hydrolysis→ZrO(NO₃),yH₂O

[0040] In the general scheme for the production of zirconium metal(zircon→carbochlorination→extractive distillation or liquid-liquidseparation), the process according to the invention can substitute forthe separation stage, with or without pretreatment at the end of thecarbochlorination.

[0041] To carry out the separation, the aqueous medium to be treated iscirculated in the vicinity of the filtration membrane and a differencein pressure is applied between the two opposite faces of the membrane.

[0042] The filtration membranes can be organic, inorganic ororganic/inorganic. They advantageously comprise or are advantageouslycomposed of polymers, such as polyaramides, sulfonated polysulfones,polybenz-imidazolones, grafted or ungrafted poly(vinylidene fluoride)s,polyamides, cellulose esters, cellulose ethers or perfluorinatedionomers, the combinations of these polymers and the copolymers obtainedfrom monomers of at least two of these polymers. For further details, aperson skilled in the art may refer to WO-A-92/06675, which disclosesorganic/inorganic nanofiltration membranes comprising an active layer ofa polymer of the polysulfone, polybenzenimidazolone, graftedpoly(vinylidene fluoride) and perfluorated ionomer (Nafion®) type-cutoffthreshold of 300 to 1 000 g.mol⁻¹; or to FR-A-2 600 264, which disclosesorganic/inorganic membranes comprising a porous and organic support anda microporous membrane made of organic polymer, such as polysulfone,polyamide, cellulose ester and cellulose ether.

[0043] Mention may be made, as examples of membranes, of the membranesmarketed by the firm Osmonics under the names of Sepa MG 17, Sepa MW-15and Sepa BQ-01, which have a permeability to double-distilled watercomprised between 2 and 10 l.h⁻¹.m⁻².bar⁻¹ at 25° C.

[0044] Use is preferably made of the tangential filtration technique,which has the advantage of limiting the phenomenon of the accumulationof the entities retained at the surface of the membrane and thus ofmaking possible continuous operation.

[0045] More preferably, use is made of filtration modules in the form oftubes or cylinders or of parallel plates or alternatively of membraneswound around a perforated tube or cylinder intended to collect thepermeate. These modules can be arranged in series and/or in parallel,with optionally different membranes in some modules.

[0046] The difference in pressure applied and the rate of circulation ofthe retentate and the temperature are adjustable parameters.

[0047] Very advantageously, it is not necessary to operate at hightemperature, as in the processes of the prior art. Even temperatureswhich might exceed the thermal resistance of the membranes used areavoided. Generally, it may be specified that the operation is carriedout at between 0 and 50° C. and more advantageously at ambienttemperature (25° C.) or in the vicinity of the latter, e.g. between 20and 35° C.

[0048] The differences in pressure and rate of circulation of theretentate are first and foremost set as a function of the desiredflowrate and of the characteristics of the membrane, e.g. its resistanceto the pressure. Simple tests make it possible to determine the optimumconditions.

[0049] However, it may be specified that the difference in pressure canadvantageously vary between 0.2 and 1.5 MPa, e.g. between 0.2 and 0.8MPa.

[0050] After its separation, the ligand-metal complexes can be treatedusing appropriate decomplexing agent(s), so as to collect, on the onehand, the ligands (which can be recycled) and, on the other hand, themetal.

[0051] According to the choice of the ligand and of the complexationconstant which results therefrom, a given metal, e.g. zirconium, isreencountered either in the retentate (case of zirconium with EDTA) orin the permeate, in the form of a complex with the ligand.

[0052] The metal in the retentate can be recovered, for example, bybasifying, e.g. to pH 10, or by evaporation.

[0053] The complexed metal can, after filtration, be released ordecomplexed, for example in a basic medium and by precipitation of itshydroxide or by passing through a specific ion-exchange resin. In thecontext of this stage, it is advantageous to provide, in accordance withthe invention, for a removal of the solvent, in this instance water, forexample by evaporation, to make possible recovery of the metal.

[0054] The apparatus required for the implementation of the processaccording to the invention is relatively limited since a complexationreactor, a pump and at least one filtration membrane, e.g.nanofiltration membrane, are sufficient. In addition, the apparatus isreadily available commercially. By way of example, the basic plant cancomprise a complexation reactor, a pump and a filtration module, e.g.nanofiltration module, e.g. tangential module, designed so that theretentate, after it has passed in the vicinity of the membrane, isrecycled upstream of the filtration module, preferably in thecomplexation reactor. According to a specific form of the invention, thereactor can be fed continuously or semicontinuously with the ligand andthe mixture of metals.

[0055] The (nano)filtration can advantageously comprise several stages,in series and/or in parallel, so as to increase the degree of separationor of enrichment, permeate and/or retentate being subjected to thenumber of treatment and (nano)filtration steps required by the objectiveto be achieved.

[0056] Likewise, successive complexing-(nano)filtration operations withidentical or different ligands can be carried out.

[0057] The invention will now be described in more detail usingembodiments taken as nonlimiting examples and with reference to thedrawing, in which:

[0058]FIG. 1 is a curve of the degree of retention in % as a function ofthe concentration of ligand EDTA, corresponding to example 2; and

[0059]FIG. 2 is a curve of the degree of retention in % as a function ofthe concentration of ligand EDTA, corresponding to example 3.

EXAMPLE 1

[0060] (Not in Accordance with the Invention):

[0061] In this example, the retention of zirconium in the form ofzirconyl dinitrate is studied by treating an aqueous solution comprising0.259 mmol/l of ZrO(NO₃)₂. A flat filtration module equipped with a SepaMG-17 membrane (with a surface area S=0.015 m ) is used. The membraneexhibits a permeability to double-distilled water of 3.6 l.h⁻¹.m⁻².bar−1at 25° C.

[0062] This study is carried out under the following ocnditions:

[0063] transmembrane pressure ΔP=0.6 MPa,

[0064] temperature=20° C.,

[0065] flowrate of retentate=40 l/h,

[0066] pH=3.6.

[0067] Several experiments were carried out by adding amounts ofzirconyl dinitrate varying from 0 to 11.7 mmol.

[0068] The results obtained show that the degree of retention of theSepa MG-17 membrane with respect to the zirconyl dinitrate is >99.99%.

EXAMPLE 2

[0069] In this example, an aqueous solution comprising 0.1 mmol/l ofzirconium and 0.1 mmol/l of hafnium in the form of zirconyl dinitrateand hafnyl dinitrate is treated. A flat filtration module equipped withthe Sepa MG-17 membrane (with a surface area S=0.015 m²) is used. Themembrane exhibits a permeability to double-distilled water of 3.6l.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexing agent composed of EDTA(Complexation constants: K_(Zr−EDTA=)10^(28.1), K_(Hf−EDTA=)10^(29.5),from where K_(Hf)/K_(Zr)=25) is added to the aqueous solution to betreated.

[0070] The Hf/Zr separation is carried out under the followingconditions:

[0071] transmembrane pressure ΔP=0.6 MPa,

[0072] temperature=20° C.,

[0073] flowrate of retentate=40 l/h,

[0074] pH=2.

[0075] Three experiments are carried out while adding the complexingagent at concentrations varying from 0 to 2 equivalent of EDTA unit perzirconium atom.

[0076] The results of the experiments are given in FIG. 1.

[0077] The results of FIG. 1 show that the degree of retention of thezirconium is greater than that of the hafnium when the solutioncomprises from 0 to 2 equivalents of EDTA units per zirconium atom. Thedifference between the degree of retention of the zirconium and that ofthe hafnium is at a maximum when the [complexing agent]/[zirconium]ratio is equal to 1. This difference then has a value of 30%.

EXAMPLE 3

[0078] In this example, an aqueous solution comprising 2 mmol/l ofzirconium and 2 mmol/l of hafnium in the form of zirconyl dinitrate andhafnyl dinitrate is treated. A flat filtration module equipped with theBP-02 membrane (with a surface area S=0.015 m²) is used. The membraneexhibits a permeability to double-distilled water of 3.6 l.h⁻¹.m⁻².bar⁻¹at 25° C. A complexing agent consisting of EDTA is added to the aqueoussolution to be treated.

[0079] The Hf/Zr separation is carried out under the followingconditions:

[0080] transmembrane pressure ΔP=0.6 MPa,

[0081] temperature=20° C.,

[0082] flowrate of retentate=40 l/h,

[0083] pH=2.2.

[0084] Several experiments are carried out while adding the complexingagent at concentrations varying from 0 to 2 equivalents of EDTA unitsper zirconium atom.

[0085] The results of the experiments are given in FIG. 2.

[0086] The results of FIG. 2 show that the degree of retention of thezirconium is greater than that of the hafnium when the solutioncomprises from 0 to 2 equivalents of EDTA unit per zirconium atom. Thedifference between the degree of retention of the zirconium and that ofthe hafnium is at a maximum when the [complexing agent]/[zirconium]ratio is equal to 1.5. This difference then has a value of 41%.

[0087] It should be clearly understood that the invention defined by theappended claims is not limited to the specific embodiments indicated inthe above description but encompasses the alternative forms which departneither from the scope nor from the spirit of the present invention.

1. A process for separating a metal 1 from a metal 2, preferablyzirconium from hafnium, starting from a solution of these metals in anaqueous medium in which these metals are in a state which does not allowthem to pass through a nanofiltration membrane, in which process theaqueous medium is treated with a ligand which complexes with the metal 1and/or with the metal 2 and then the medium, thus treated, is passedover a filtration membrane which allows the ligand-metal complexes topass but which retains the metals not complexed with the ligand.
 2. Theprocess as claimed in claim 1, characterized in that the metals arezirconium and hafnium in solution in an acidic aqueous medium.
 3. Theprocess as claimed in claim 2, characterized in that the metalszirconium and hafnium are in solution in an acidic aqueous medium havinga pH of less than or equal to
 4. 4. The process as claimed in claim 3,characterized in that the pH is comprised between 1 and 4, preferablybetween 2 and
 4. 5. The process as claimed in any one of claims 1 to 4,characterized in that the ligand complexes with zirconium or withhafnium.
 6. The process as claimed in any one of claims 1 to 4,characterized in that the ligand complexes with zirconium and withhafnium with different complexation constants and in that a firstfraction, comprising the ligand/metal complex having the highestcomplexation constant, is recovered by filtration.
 7. The process asclaimed in any one of claims 1 to 6, characterized in that the ligand isa poly(amino acid).
 8. The process as claimed in one of claims 1 to 6,characterized in that the ligand is a cyclic poly(amino acid).
 9. Theprocess as claimed in any one of claims 1 to 6, characterized in thatthe ligand is a poly(amino acid) of formula (1):

in which: n=0 to 3, m=1 to 4, preferably m=2, X and Y are identical ordifferent and represent hydrophilic radicals of OH or NR¹R² type, withR¹ and R², which are identical or different, each corresponding tohydrogen or to a hydrophilic monovalent radical preferably selected fromaminated and/or (poly)hydroxylated and/or (poly)etherifiedhydrocarbonaceous residues, these residues preferably being of thecycloalkyl, aralkyl, alkylaryl, cycloalkenyl, aralkenyl, alkenylaryl oraryl type.
 10. The process as claimed in claim 9, characterized in thatthe ligand is EDTA.
 11. The process as claimed in claim 9, characterizedin that the ligand is diamido-EDTA.
 12. The process as claimed in claim8, characterized in that the ligand is DOTA.
 13. The process as claimedin any one of claims 1 to 12, characterized in that the amount of ligandadded is such that the ligand concentration/concentration of metalhaving the strongest complexation constant ratio is between 0.5 and 2,preferably between 0.8 and 1.7.
 14. The process as claimed in claim 13,characterized in that the ratio is comprised between 0.9 and 1.6. 15.The process as claimed in any one of claims 1 to 14, characterized inthat the filtration membrane is a nanofiltration membrane.
 16. Theprocess as claimed in any one of claims 1 to 14, characterized in thatthe filtration membrane is an ultrafiltration membrane.
 17. The processas claimed in any one of claims 1 to 16, characterized in that theprocess is carried out at a temperature comprised between 0 and 50° C.18. The process as claimed in claim 17, characterized in that thetemperature is comprised between 20 and 35° C. and preferably ambienttemperature.
 19. The process as claimed in any one of claims 1 to 18,characterized in that the metals are in solution in the aqueous mediumin the form of salts of chlorate, perchlorate or nitrate type.
 20. Theprocess as claimed in claim 19, characterized in that zirconium andhafnium are in the form of MOCl₂ or MO(NO₃)₂ salts, with M=Zr or Hf. 21.The process as claimed in any one of claims 1 to 20, characterized inthat the process is applied to a ZrCl₄+HfCl₄ mixture.
 22. The process asclaimed in any one of claims 1 to 20, characterized in that the processis applied to a ZrO₂+HfO₂ or ZrCl₄+HfCl₄ mixture which has beensubjected to treatment with nitric acid.